This invention relates to rapid screening for activities and selectivities of heterogeneous and homogeneous catalyst libraries by mass spectrometry. This invention provides very rapid screening of gaseous, liquid or solid products from all catalyst sites in a catalyst library by mass spectrometry and its combination with selective resonance enhanced multiphoton ionization (REMPI).
Solid and liquid catalysts are used in the manufacture of a vast array of chemicals and fuels, and in this manner significantly contribute to the economy and high living standards. National Research Council, xe2x80x9cCatalysis Looks to the Futurexe2x80x9d, National Academy Press, Washington, D.C., 1992. Catalysts also provide important environmental benefits, such as in catalytic converters for internal combustion engines. However, in spite of their significance and broad utility, the development of new and improved catalysts continues to be an arduous and rather unpredictable trial and error process. Conventionally, an individual catalyst is prepared using a large variety of tedious and time consuming methods, characterized and tested for catalytic activity, modified, again characterized and tested again, until no further improvements are justified. This approach, although time consuming, has been successful for the discovery of a significant number of solid state catalysts, Heinemann, H., xe2x80x9cA Brief History of Industrial Catalystsxe2x80x9d, Catalysis: Science and Technology, Anderson, J. R. and Boudart, M. Eds., Chapter 1, Springer-Verlag, Berlin, 1981, and homogeneous, liquid-state catalysts, Montreus, A. and Petit, F., xe2x80x9cIndustrial Applications of Industrial Catalystsxe2x80x9d Kluwer Publishing, New York, 1988.
Combinatorial chemistry, in which a large number of chemical variants are produced rapidly and a chemical library generated which is then screened for desirable properties using a suitable technique, is a particularly attractive approach for the discovery of new catalysts. Chem. Eng. News, Feb. 12, 1996. Combinatorial synthesis was initially used to synthesize large libraries of biological oligomers, such as peptides and nucleotides, however, the creation of small molecule libraries which can be used for drug testing is growing. Nielsen, J., Chem. and Indus., 902, Nov. 21, 1994. Recently, combinatorial diversity synthesis has been extended to solid-state compounds used in superconducting, xiang, X-D., Sun, X., Briceno, G., Lou, Y., Wang, K-A., Chang, H., Wallace-Freedman, W. G., Chen, S-W. and Schultz, P. G., xe2x80x9cA Combinatorial Approach to Materials Discoveryxe2x80x9d, Science, 268, 1738, 1995, magnetoresistivity, Briceno, G., Chang, H., Sun, X., Schultz, P. G. and xiang, X-D., xe2x80x9cA Class of Cobalt Oxide Magnetoresistance Materials Discovered With Combinatorial Synthesisxe2x80x9d Science, 270, 273, 1995 and luminescence, Wang, J., Yoo, Y., Takeuchi, I, Sun X-D., Chang, H., Xiang, X-D. and Schultz, P. G., xe2x80x9cIdentification of Blue Photoluminescent Composite Material from a Combinatorial Libraryxe2x80x9d, Science 279, 1712, 1998, Danielson, E., Golden, J. H., McFarland, E. W., Reaves, C. M., Weinberg, W. H., and Wu, X-D., xe2x80x9cA Combinatorial Approach to the Discovery and Optimization of Luminescent Materialsxe2x80x9d, Nature, 398, 944, 1997, Sun, X-D, Gao, C., Wang, J. and Xiang, X-D., xe2x80x9cIdentification and Optimization of Advanced Phosphors using Combinatorial Librariesxe2x80x9d, App.Phy.Lett., 70, 3353, 1997 and Sun, X-D., Wang, K. A., Yoo, Y., Wallace-Freedman, W. G., Gao, C, xiang, X-D. and Schultz, P.G., xe2x80x9cSolution-Phase Synthesis of Luminescent Materials Librariesxe2x80x9d, Adv.Mater, 9, 1046, 1997. In these cases physically masked individual specimens were each measured using contact probes with a computer-controlled multichannel switching system. Microprobe sampling coupled to mass spectrometry, Kassem, M., Qum, M. and Senkan, S. M., xe2x80x9cChemical Structure of Fuel-Rich 1,2-C2H4Cl2/CH4/O2/Ar Flames: Effects of Microprobe Cooling on Sampling of Flames of Chlorinated Hydrocarbonsxe2x80x9d, Combust. Sci. Tech., 67, 147, 1989, and in situ IR, Moates, F. C., Somani, M., Annamalai, J., Richardson, J. T., Luss, D. and Wilson, R. C., xe2x80x9cInfrared Thermographic Screening of Combinatorial Libraries of Heterogeneous Catalystsxe2x80x9d, Ind. Eng. Chem. Res., 35, 4801, 1996, have been proposed for catalyst screening, but suffer serious deficiencies in not having sufficient sensitivity, selectivity, spatial resolution or high throughput capacity to screen large catalyst libraries, as well as the lack of ability to test the activity of hundreds or thousands of compounds simultaneously. Service, R. F., xe2x80x9cHigh Speed Materials Designxe2x80x9d, Science, 277, 474, 1997. Microprobe mass spectrometry requires sampling and transfer of very small quantities of gases containing low concentrations of product species from each site rendering the process impractical for rapid screening. In situ infrared techniques cannot provide information on product selectivity which is crucial for catalyst identification.
Mass spectrometry is a well established and broadly applicable method for determining mass of gaseous species. The technique involves the ionization of gaseous molecules by a number of methods, such as, for example, by electron impact or light photoionization followed by separation of ions using techniques, such as, for example, quadrupole mass spectrometry or time of flight mass spectrometry and detection of selected ions by a suitable detector. Capillary probe sampling mass spectrometry has recently been reported for screening of catalyst libraries by Cong, P.; Giaquinta, D.; Guan, S.; McFarland, E.; Self, K.; Turner, H.; and Weinberg, W. H., xe2x80x9cA combinatorial Chemistry Approach to Oxidation Catalyst Discovery and Optimizationxe2x80x9d, Process Miniaturization Section, 2nd Intl. Conf. Micro Technol., Mar. 9-12, 1998, New Orleans, La., pg. 118. Cong, et al teach introduction of reactant gas to an individual library site through an annular space surrounding a capillary tube through which product gas flows from that library site to the ionization zone of a mass spectrometer. Cong. et al report measurement of 144 library sites in about 2 hours. Sample transfer rates by capillary in the Cong, et al method are limited by the pumping speed tolerated by the mass spectrometer chamber. Another disadvantage of capillary probe sampling is the potential of adsorption and catalysis induced by relatively long transfer line surfaces. There remains a large unexplored universe of binary, ternary, quaternary and higher-order solid state materials, organometallic species and other complex metal compounds that could have superior catalytic properties. Prior conventional approaches have been inadequate to rapidly synthesize and screen this vast universe of catalytic compounds. There is clearly a need for development or more efficient and systematic methods to produce heterogeneous and homogeneous state libraries and to screen them for desired catalytic properties. Combinatorial solid state synthesis techniques have not been applied to the discovery of new and/or improved catalysts. A significant impediment for this has been the lack of a broadly applicable, sensitive, selective and high throughput measurement technique which could be used to rapidly screen large catalysts libraries. Catalyst screening requires the unambiguous detection of the presence of a specific product molecule in the vicinity of a small catalyst site on a large library, unlike superconductivity or magnetoresisitivity which can both be easily tested by conventional contact probes, or luminescence that can be tested by light emission.
This invention provides a high-throughput method to rapidly screen the activities and selectivities of homogeneous and heterogeneous catalyst libraries generated by combinatorial synthesis. Solid and liquid state catalyst libraries can be generated using a variety of techniques and can involve the combination of a large number of chemical elements and compounds.
In one embodiment, catalyst libraries may be screened for both activity and selectivity by high throughput screening using mass spectrometry. Catalyst libraries of microreactors and direct transfer of reaction products to a mass spectrometer for analysis according to this invention provides rapid screening of catalyst libraries. The technique and apparatus of this invention using catalyst libraries of an array of microreactors in monolithic structures with free jet sampling probes passing reaction products to a mass spectrometer makes it possible to screen each site in about one to five seconds, a significant improvement over the teachings of the Cong, et al reference cited above, while eliminating potential wall effects inherent in capillary microprobe sampling.
In another embodiment, the mass spectrometric analysis may also be used in combination with resonance-enhanced ionization of product gases and microelectrode screening. In cases where both screening methods are feasible, radiation activation may be used to rapidly identify promising sites and then mass spectrometry may be used to quantify yields and selectivities in greater detail. In instances in which the identification of radiation frequencies over which unique resonance enhanced multiphoton ionization signals of reaction products may not be feasible, the mass spectrometric method may be used to rapidly screen. catalyst libraries.
Detection methods in situ in the reactor use the high sensitivity, specificity and real-time features of resonance-enhanced multiphoton ionization, REMPI, in which pulsed and tunable ionizing light sources are used to selectively photoionize desired reaction products without ionizing reactants and/or other background species. Photoions or photoelectrons generated by a tunable light beam in a reaction product plume from reactants in contact with a specific catalyst library site are detected by an array of microelectrodes positioned in close proximity to the library sites. While this invention will be described using a tunable ionizing beam, any radiation beam of an energy level to promote formation of specified photoions and photoelectrons may be used. When reaction products are solids or liquids, they can be ablated using a pulsed laser beam followed by selective photoionization of the products- using a suitable UV laser. The process of this invention can provide information on catalyst selectivity by detecting several reaction product species. This can be done using different light frequencies to sequentially generate specific ions of different products and the REMPI signals can then be converted into absolute concentrations by use of calibration standards.
Internal calibration standards introduced with the reactant feed can be used to quantify reaction products, as will be readily apparent to one skilled in the art. The process of this invention is broadly applicable and can be used to simultaneously screen an entire catalyst library. The process of this invention can also be used to study operational lifetimes, resistance to poisoning, regeneration and loss of catalysts in tests or in full scale chemical plant processes.
The process of this invention for rapid screening of potential catalyst libraries for catalytic properties broadly comprises; forming a potential catalyst library having potential catalysts at a plurality of addressable sites, passing reactant gas in contact with the potential catalysts at the plurality of addressable sites, and screening gas plumes of products of reaction from the addressable sites, the screening comprising at least one of translating one of the addressable sites into a position in proximity to a sampling probe orifice followed by passing products of reaction through a free jet sampling probe to a mass spectrometer for analysis and passing a radiation beam of an energy level to promote formation of specified ions and electrons in the product stream, such as, for example, a laser beam of a frequency to promote formation of specified photoions or photoelectrons and detecting the formed photoions or photoelectrons by microelectrode collection in situ in proximity to the addressable sites.